One type of portable rock drill rig is generally characterized by a crawler undercarriage upon which is mounted an elongated mast or feed beam for slidably supporting a drill motor. The mast is adapted through its support arrangement to be pivoted about one or more pivot axes whereby drilling in various directions may be carried out and the mast may be positioned for ease of transport from one work site to another.
The ever increasing need for drilling deeper holes in one pass of the drill along the mast has resulted in mast lengths which are so great as to make it difficult to provide mounting and positioning mechanisms which will provide a suitable range of mast positions for drilling and permit lowering of the mast to a suitable position for transporting the rig in a stable condition requiring minimum vertical clearance.
U.S. Pat. Nos. 3,322,378 and 3,529,679 disclose mast mounting arrangements for drilling apparatus of the general type to which the present invention pertains.